It is thought that the Ro-scull which is a traditional manually rowing apparatus was brought to Japan from China at around a time before the Kamakura period. Since the Ro-scull was brought to Japan, the Ro-scull had been gradually improved, and the final form of the Ro-scull was completed in the early Edo period. Then, the form of the Ro-scull has been kept in substantially the same shape.
The Japanese Ro-scull has two features: (1) two rods of materials are used while being joined together; and (2) the two rods of materials thereof are joined to form a bent configuration.
Particularly, the Ro-scull having the above features is called as “Tsuguro (joined Ro-scull).” On the other hand, the Ro-scull in which two rods of materials are not used is called as “Saoro (rod Ro-scull).”
FIG. 6 shows a structure of the conventional Ro-scull.
The conventional Ro-scull consists of two large parts and two small parts. Referring to the perspective view of FIG. 6, each structure will be described.
In FIG. 6, the reference numeral 101 denotes a Roasi (hereinafter, Ro-blade or yuloh blade) which paddles the boat, and the Ro-blade 101 has a spatula-shaped flat part 110. The reference numeral 102 denotes a Roude (hereinafter, Ro-arm or yuloh arm) which is rigidly fixed to the Ro-blade 101 so as to be substantially horizontally held when the flat part 110 is orientated to an obliquely upward direction. Near a joint portion between the Ro-blade 101 and the Ro-arm 102, there is no flat part existing, but there is a part 120 (usually known as Ireko (insert)) having a round shape. A user puts the part 120 on a shaft support part 201 (usually called as Robeso (tholepin) or Rogui (Ro-stake or yuloh stake) provided at a rear end portion of a boat 200, (or the part 120 is rotatably supported by the shaft support part 201). The operator of the boat operates the Ro-arm 102 from side to side, thereby moving the Ro-blade 101 from side to side with the shaft support portion working as a pilot.
A small projected Rozuka (Ro-handle or yuloh handle) 103 is rigidly fixed onto the upper surface of the Ro-arm 102 and the Ro-handle 103 is used with a rope called Hayao 104 being tied thereto. The other end of the Hayao 104 is rigidly fixed to the bottom side of the boat, and the Hayao has a function of transmitting a thrust force to the boat when the thrust force is generated while the Ro-scull is operated.
The action of the conventional Ro-scull having the above structure will be described below.
The operator operates the Ro-scull arm 102 from side to side so that the flat part of the Ro-blade 101 is inclined with respect to an advancing direction. FIG. 7 shows the movement of a cross section of the Ro-blade 101 in the time-series order at the position where the Ro-blade is in contact with a water surface when the operator operates the Ro-scull. Assuming that the advancing direction of the boat is the lower side of the drawing, a to c in FIG. 7 show the states each in which the Ro-blade 101 is moved leftward with respect to the advancing direction of the boat, i.e. a to c in FIGS. 7 show a transition when the operator moves the Ro-arm 102 rightward. In the cross section of the Ro-blade 101, the sign f denotes a front edge in the advancing direction of the Ro-blade 101, and the sign r denotes a rear edge in the advancing direction of the Ro-blade 101.
A water flow generated in such a case relative to the Ro-scull is shown by a water flow 300 in (a) of FIG. 8.
As can be seen from the foregoing figure, a difference of flow in the water flow is generated between the top surface and the bottom surface of the flat part 110 of the Ro-blade 101 by obliquely moving the Ro-blade 101. The difference in the water flow creates a force similar to the force called “lift force” in an aircraft and the like, whereby, a thrust force in a direction of an arrow 400 is generated. When, thereafter, the movement of the Ro-blade 101 is changed from the leftward to the rightward with respect to the advancing direction, namely, when the moving direction of the Ro-arm 102 is changed, the Ro-blade is moved as shown in d to f of FIGS. 7.
In this case, a water flow 301 is created as shown in (c) of FIG. 8, and, as expected, the thrust force is generated in the direction of an arrow 401 similar to the direction of the arrow 400.
At a point in which the moving direction of the Ro-blade 101 is changed from the left to the right (the point between c and d of FIG. 7), it is necessary that the inclination of the Ro-blade is reversed (usually called as Kaeshi (turn-over)).
As can be seen from the above, among the manually rowing methods such as a paddle and an oar, the Ro-scull is the most functional in that the hydrodynamic lift force is used as the thrust force.
In an ideal condition, it is known that the lift force (thrust force) generated in the above-described manner is ten times as large as the drag force generated. Namely, the lift is generated ten times the rowing force.
Although the lift force is transmitted as a thrust force to a stern, the operator does not sense the thrust force by an operator's arm because the Hayao 104 and the fulcrum of the Ro-scull receive the thrust force. Further, unlike other manually rowing apparatuses, the Ro-scull has no wasted motion because the thrust force is generated in both directions of the reciprocating motion.
However, in the conventional Ro-scull, the flat part 110 obstructs the water flow at the point of the turn-over. In this case, as shown in (b) of FIG. 8, the water flow orthogonally hits the flat part 110 of the Ro-blade 101, so that resistance caused by the water flow is largely increased. In addition, large vortexes 302 are generated on the downstream side of the water flow, which results in the decrease in the thrust force, and whereby, the thrust efficiency is remarkably decreased.
In particular, because the vortexes are radically generated with the increasing speed of the boat, the thrust efficiency worsens as the speed of the boat is increased, and actual high-speed cruise by the thrust by the Ro-scull becomes difficult.
Therefore, in the case of the manually rowing boat with the Ro-scull, there is a problem that the speed of the boat becomes slower when compared with the oar which generates the thrust force on the side of the boat.